Bio-oil Levoglucosan Production
Levoglucosan (1,6-anhydro-β-D-glucopyranose) is a product of cellulose combustion. When cellulose is heated to over 300° C., it undergoes various pyrolytic processes to produce as a major component in the resultant bio-oil an anhydrosugar known as levoglucosan. Levoglucosenone, in addition to levoglucosan, and other various anhydrosugars are produced in lesser but in additively significant quantities. Whole pyrolysis oil contains approximately 3% of levoglucosan when untreated wood biomass is pyrolyzed. Researchers have extensively investigated the mechanisms underlying levoglucosan formation during pyrolysis but a full understanding of the formation, intermediate reactions and degradation remain unknown. There are, however, empirical studies that have been performed demonstrating methods to increase anhydrosugars yield in bio-oil.
Shafizadeh (1980) analyzed the production of levoglucosan from various cellulose types and cottonwood wood fiber. His results were given as percentage yields of each tested feedstock. At 400-450° C. the range of yields of levoglucosan plus anhydrosugars from four pure commercial wood-based cellulose types was 42 to 56%. Yields of levoglucosan from wood, as opposed to pure cellulose, are known to be lower and Shafizadeh's results confirmed this with yields of levoglucosan plus anhydrosugar of 3% for untreated cottonwood fiber and 9% for H2SO4 acid-treated cottonwood fiber.
Complete removal of hemi-cellulose to produce a pure lignocellulose was performed by Shafizadeh (1980) and resulted in 37% yield of levoglucosan plus anhydrosugars. This researcher also pyrolyzed untreated pure holocellulose (lignin removed) to obtain only a 1% yield of levoglucosan and anhydrosugars. When the holocellulose was treated with 1% NaOH and 1% H2SO4 the yield of levoglucosan plus other anhydrosugars increased to 32%. The removal of hemi-cellulose resulted in a large increase in levoglucosan and other anhydrosugars yield. These results indicate that presence of hemicellulose rather than lignin in wood interacts to inhibit levoglucosan and anhydrosugars yields during pyrolysis.
Piskorz et al. (1989) studied fluidized bed fast pyrolysis of poplar wood and a number of types of cellulose produced by different processes. Both untreated and treated cellulose and poplar wood specimens were pyrolyzed at 500° C. with vapor residence time of 0.46 s. Biomass treatment was mild acid treatment at 5% of H2SO4 at 90° C. for 5.5 hours. Levoglucosan and anhydrosugars were quantified in the resultant bio-oil with a yield of 43.5% of these compounds for untreated cellulose and a somewhat higher 53.1% yield for cellulose with mild acid treatment. In addition, untreated poplar wood, which would contain all of its initial hemicelluloses, produced total levoglucosan plus anhydrosugars yield of only 8.95% while mild acid treated poplar wood produced a total yield of these compounds of 40.61%.
Piskorz et al. (1989) also tested, on poplar wood, a milder application of sulfuric acid and two mild acid HCl treatments with one of the mild HCl treatments followed by an additional H2SO4 treatment. Anhydrosugar yields were measured only in terms of levoglucosan yields. The results of these treatments indicated that levoglucosan yields from HCl treatments were a relatively low 4.51 and 17.5, with the lowest yield for the mildest HCl treatment. The highest yields of levoglucosan were 27.7 and 30.1% for the H2SO4 treatment alone and the H2SO4 treatment following HCl treatment, respectively. The treatment described above for 5% H2SO4 at 90° C. for 5.5 hours provided a levoglucosan yield of 30.42% while the second H2SO4 treatment was at 5% concentration for 2 h heating time at 100° C.
Research has also shown that deionized water or deionized hot water treatment sufficiently demineralizes biomass to increase anhydrosugars content (Scott et al. 1995). Therefore, water, or hot water deionization is in the public domain and may be practiced to increase anhydrosugars yields in bio-oil in conjunction with the utilization of our device and method.
Increased levoglucosan yields from various applied pretreatment processes to switchgrass feedstocks were investigated by Brown et al. (2001) to determine influence on yields of levoglucosan and cellobiosan. These researchers computed yields based on percentage of the aqueous bio-oil fraction. Three pretreatment processes were evaluated: acid hydrolysis (5.0 wt % H2SO4 at 100° C. for 2 h), demineralization that consisted of washing in dilute nitric acid (0.25% nitric acid at 25° C.), and demineralization with addition of 0.1% (NH4)2SO4 as catalyst. After pyrolysis at 430° C. to 450° C. the concentration of levoglucosan in the organic fraction of untreated switchgrass pyrolysolate was only 4.4% with cellobiosan yield undetectable. These values increased to 35.6% of levoglucosan and 3.27% cellobiosan yield for a total of 38.87% yield of levoglucosan plus anhydrosugars for H2SO4 hydrolyzed switchgrass, 16.1% for demineralized switchgrass, and 20.7% for demineralized switchgrass with ammonia sulfate catalyst added.
Brown et al. (2001) also compared the same treatments for corn stover. The concentration of levoglucosan plus cellobiosan in the organic fraction of raw corn stover pyrolysolate totaled only 2.8%, increasing to 28.1% for hydrolyzed corn stover, 22.2% for demineralized corn stover, and 23.5% for demineralized corn stover with ammonia sulfate catalyst added.
Scott et al. (1995) received a patent on a method for pretreating cellulose or wood biomass with mild acids at various concentrations. Following pretreatment, feedstocks were washed with deionized water and dried to 1 to 30% moisture content followed by fast pyrolysis. Anhydrosugars were fractionated from the bio-oil and crystallized by various means. Water fractionation to separate the anhydrosugars into the aqueous fraction was not one of these means, however. Yield of levoglucosan from 99% pure cellulose was 45.7%; from hybrid poplar wood the levoglucosan yield was estimated at 35 to 40%; western hemlock wood was pretreated by mild acid, drained and dried to give 18.7% levoglucosan yield.
Bennett, et al. (2009) examined the potential to produce levoglucosan from bio-oil without biomass demineralization or hydrolysis to remove hemicellulose. Rather, the ratio of water added to achieve fractionation varied to determine an optimum level. For the optimum water addition level of 40% the yield of levoglucosan was 7.8% of total raw bio-oil weight. These researchers did not utilize acid pretreatment or water spray into the pyrolysis vapor stream to increase the yields of anhydrosugars with the expected result that their yields are considerably lower than possible with our device and methods.
The results reviewed above show that hot water or mild acid pretreatment of cellulose or lignocellulosic biomass, washing the biomass with distilled water, drying to a moisture content suitable for pyrolysis and application of fast pyrolysis has produced elevated yields of anhydrosugars in bio-oil compared to yields of untreated biomass. Yields vary but for pure untreated cellulose levoglucosan yields are about 3% while the yields for mild-acid pretreated cellulose the yields frequently exceed 50%. For lignocellulosic biomass the levoglucosan yields for untreated feedstocks is also about 3% and for treated material the highest reported yield is 36.3% for switchgrass feedstock. Spraying liquid or injecting a cooling gas into the pyrolytic vapor stream to reduce decomposition of the pyrolytic anhydrosugars during condensation was not mentioned by any previous practitioners.
Ultrasonic Lignocellulosic Biomass Pretreatment
Application of ultrasound technology to achieve biomass cellular disintegration to increase material surface area has been recently described in a thorough review of the technology application to the food industry (Vilkhu et al. 2008). The cellular disintegration resulting in size reduction combined with intra-particle diffusion are the mechanisms that were identified as allowing increased access by solvents, and/or catalysts, to the sonicated cellular material. This increased access appears to be responsible for the increased rates of chemical product extractions observed by practitioners. Researchers have employed ultrasound to lignocellulosic biomass feedstocks to improve the extractability of hemicellose (Ebringerova et al. 2002), cellulose (Pappas et al. 2002), and lignin (Sun et al. 2002; Fengel et al. 1984) or to produce cellulosic fiber from used paper (Scott et al. 1995; Sell et al. 1995).
Toma et al. (2006) employed ultrasound to improve the enzymatic hydrolysis of lignocellulosic materials in a two-stage process. First-stage sonication was applied to increase biomass surface area through cellular disintegration to smaller particles with increased surface area. Second-stage ultrasound was then applied to the pretreated biomass to increase hydrolysis rate during enzymatic treatment. Direct sonication at 20 kHz increased glucose yield by 93%.
A method comprising applying ultrasonic energy to a biomass to increase alcohol production rates and yields has been disclosed by Kinley and Krohn (2005). This invention employed the use of ultrasonic energy as a pretreatment step, either alone or in combination with any conventional pretreatment methods. The main objective of the Kinley and Krohn (2005) patent was to improve the efficiency of conventional ethanol production via various potential sonication treatments. Following conventional reduction of feedstock particle size by grinding the biomass water is added to produce slurry that is then sonicated in a pretreatment process to produce further particle size reduction and disaggregation of cellular structure. During this treatment a mild hydrolysis of a portion of the cellulose and hemicellulose components is claimed by the inventers. A second sonication is then applied to improve the typical acid-catalyzed hydrolysis performed on biomass during ethanol production. This treatment employs acid at a concentration necessary “to hydrolyze the hemicellulose fraction and decrystallize the cellulose into an amorphous state.” Alternatively, the hydrolysis treatment may be achieved by enzyme catalysis also energized by sonication treatment (Kinley and Krohn 2005).
Kinley and Krohn (2005) performed only limited experiments to prove their process. Pretreatment of switchgrass slurry with only water added was sonicated for 0, 5 and 15 min followed by ethanol production from the slurry by simultaneous saccharification and fermentation or 7 days. Results showed that ethanol efficiency increased from 11.6 to 24.4% for 5 min sonication treatment but declined to 22.8% for the 15 min sonication treatment. In a second experiment the inventors found that 15 min of sonication of a pretreated biomass slurry rendered cellulose about 21-24% more digestible. In none of the references to sonication do practitioners mention that their technology was applied to biomass feedstocks to reduce the reaction time or to reduce the catalyst required by a reaction for demineralization.
Fast-Pyrolysis Quenching Technology
Fast pyrolysis technology produces a hot vapor stream that must be rapidly condensed by cooling. Some practitioners have applied quenching solids or liquids to achieve the required rapid cooling of the pyrolysis vapor. The earliest discussion of cooling pyrolysis vapor by quenching was by Sack (1978). By this method a solid carbonaceous material is utilized to rapidly cool pyrolysis vapors. The hot pyrolysis vapors act to preheat the carbonaceous materials prior to their pyrolysis. In addition to the preheating of the carbonaceous materials by the hot pyrolysis vapors a portion of the heavier hydrocarbons are condensed in the quench zone.
Freel et al. (1998) described a pyrolysis system for production of bio-oil by a circulating fluidized bed system. The pyrolysis vapors produced by this system are quenched for rapid cooling by cooled recycled bio-oil or by “other liquid solvent”.
Conroy and Verma (1999) disclose a means for olefin fractionation via fast pyrolysis. Pyrolysis vapor from the decomposed feedstock is partially cooled by quenching with cooled bio-oil produced during the pyrolysis process. The quenching liquid can be liquid olefins or water. No practitioner has described injection of a cooling gas into the pyrolysis vapor stream to increase production of anhydrosugars in bio-oil.
Fractionating Condenser Technology
Scott et al. (1995) disclose a process for the production of anhydrosugars from pyrolytic bio-oil. These inventors do not mention partial or complete factional separation of aqueous, or pyroligneous fractions. Boateng et al. (2007) discuss the design of a condenser system for a fluidized bed pyrolysis system utilizing an impinger-type condenser with multiple canisters cooled by chilled water bath. Final pyrolysis vapor is collected with an electrostatic precipitator. No mention is made of employing differential temperatures to fractionally separate aqueous or pyroligneous fractions.
Agblevor (2009) discloses a catalytic fractionation system for condensation of pyrolysis vapor produced by a catalytic fluidized bed reactor by various cooling media including chilled water condensers, electrostatic precipitators, coalescence precipitators, coalescence filters and combinations thereof. The fractionated slate of products claimed by Agblevor (2009) are phenols, cresols, catechols, guaiacol, methyl-substituted phenols, indene and substituted naphthalene, syngas, char, and coke solids, c1-c4 hydrocarbons. No mention is made of concentrating a high proportion of an anhydrosugar-rich aqueous fraction. Water is not sprayed into the pyrolysis vapor stream to reduce anhydrosugar decomposition and neither levoglucosan nor other anhydrosugars are referenced as fractionated products. For the invention that we describe here we claim precedence over the Agblevor method as our conception and demonstrated application preceded his by several years.
Brown (2009) describes an auger reactor design utilizing multiple condensers to rapidly condense pyrolysis vapors. For the invention that we describe here we claim precedence over the Brown (2009) method as our conception and demonstrated application preceded his by several years. The Brown (2009) device's condenser train is comprised of three water condensers and an electrostatic precipitator which we shall reference as Condenser 4 to simply our discussion. Condensers 1 and 2 are followed by the electrostatic precipitator (Condenser 4) that acts as a condenser. Condenser 3 follows the electrostatic precipitator. The differential cooling of the condensers acts to condense various fractions of bio-oil depending on relative condenser temperature and molecular weight of the fraction. The four bio-oil fractions obtained from the respective Condensers 1 to 4 were analyzed for water content. The mean the described runs produced approximately (as estimated from graphical results) 17, 41, 18 and 66% water content values for Condensers 1, 2, 3 and 4, respectively. These results indicate that the multiple condensers employed for this device act to produce high water content fractions for two of the condensers, particularly for Condenser 4. However, high water content, without substantial bio-oil or levoglucosan yield is not an advantage in concentrating anhydrosugars yields during pyrolysis.
Brown (2009) provides levoglucosan yields for each of his bio-oil collection points. These results are for feedstocks that were not pretreated with acid and for which the vapor stream was not sprayed as described as one novelty of our patent. Therefore, the levoglucosan yields reported by Brown (2009) are low as would be expected for untreated biomass and for a reactor in which water is not sprayed into the pyrolysis vapor stream. Brown (2009) reports that levoglucosan yields were 2.246, 1.333, 2.244 and 0.000 for Condensers 1, 2, 3 and 4, respectively. These results show that the respective percentage yields of levoglucosan in Condensers 1, 2, 3 and 4 as a fraction of total levoglucosan produced were 58.9%, 19.1%, 21.9% and 0.0%. These percentage yields indicate that the collection of levoglucosan by the Brown (2009) condensers is relatively constant across all condensers with the exception of Condenser 4 which produced a value of zero levoglucosan. Brown (2009) discloses no method for increasing anhydrosugars in bio-oil nor does he demonstrate a method to concentrate a large percentage of bio-oil water content with its rich levoglucosan content in a single condenser.
Pyrolysis of High-Water Biomass
There is considerable effort being expended by researchers to develop means to pyrolyze high-moisture biomass in order to reduce the cost of drying to approximately 10% water content, or less. Little success has been reported to date by other researchers. However, we have tested pyrolysis of high moisture content biomass and have successfully pyrolyzed feedstocks with up to 50% water content. A major problem with this pyrolysis for most systems is the production of a large amount of water in the final bio-oil. However, our novel system of condensation of much of the pyrolysis water produced from pyrolysis of high-moisture feedstocks into a single condenser by our method will allows production of raw bio-oil with acceptable (<30%) water content under the condition that the bio-oil collected in the single condenser is treated. This treatment requires, as is described in this disclosure, allowing the self fractionation of the high water content bio-oil followed by removal of the anhydrosugar-rich aqueous fraction. The remaining pyroligneous fraction can then be combined with the bio-oils in the remaining condensers to produce a relatively low water content bio-oil. Researchers have previously avoided production of high water content bio-oil. This bio-oil will self fractionate at a water content between much above 40%. Disposal of this chemically-rich product has previously simply been an expense that has outweighed the cost of feedstock drying costs. However, our novel spraying and condenser fractionation device and methods are able to produce pyrolytic anhydrosugars in this water fraction that will more than justify the cost of water production during pyrolysis. Our method and device changes what is an unacceptable expense into a source of income. This water can be utilized as an anhydrosugar-rich substrate that can be hydrolyzed to simple sugars for biochemical fermentation or for reforming to hydrogen or hydrocarbons.